AD8421 output oscillation with no input signal

The inputs (left side) are small DC voltages, within ~100mV of ground. They won't be DC in the final application, but for debugging that's what they are. No gain resistors are used. There are decoupling caps on the +/- 15V power supply (0.1uF ceramic 0402 close to pins, 10uF tantalum 1206 next, then 1mF electrolytic ~3cm from pins). It's a lab power supply, very clean.

The output signal is a very clean oscillation ~1V peak to peak, ~6 MHz. Shown below are the inputs (traces 3 and 4, purple and pink respectively), and the output in green.

When I add a 10nF cap in parallel with the 1kOhm output resistor, R5, I get the trace below, with a ~2MHz frequency but similar amplitude.

I added a 10uF ceramic capacitor in parallel with R5, and the oscillations were either small enough that I couldn't see them on the scope any more or they were gone entirely. I may be able leave the output cap in my final circuit, but I would be much more comfortable with doing so if I understood where this problem was coming from.

Thanks for the quick reply. It's a 4 layer board with a ground plane below the signal layers in use for this circuit.

I tried several different values of Rg, including 10k, and spanning below 470 (can't remember exactly what value) and saw no effects. I hadn't tried larger values of RL before, but very small (10 Ohm) RL made the oscillations very small or non-existent. I'm concerned that such a small output resistance will cause the chip to output too much current at high voltages and burn it out.

I just tried 10k and open for RL and observed no change in amplitude or frequency of the oscillations. I used a variable resistor to reduce the resistance from 1k down to ~10 ohms and found the turn off to be around 50 Ohms.

Are you using your own design fabrication board? How many boards do you have that show this kind of problem?

I don't see any obvious error on your design and therefore what I would do first is to double check the connections of your board and your setup.

From your scope shot, does the purple and pink is measured directly at input pins (top of R2 and R4)?

Can you verify if you have correct supply at the power pins of the AD8421 (Pin 5 and Pin 8)? Just want to make sure that you have good connection and you soldered the part properly. Please also check the voltage across REF pins.

If you get good reading and you think you have good connections, I would still make a simple DC test just to confirm that all traces is really connected/soldered properly (i.e power supply, ref, input pins) and the AD8421 is not yet damage.

I would remove R1, R2, C2, RL. Replace C1 with 0 ohm or you can just short it by solder bridge. And put 1V DC at +Input connector. Check if you get 1V DC at the output. If it is still oscillating, check again the voltage directly across the input pins, supply pins and reference pin. If all has a correct voltage but the output still oscillation then you might damage the part and try to replace it with new part and see if it removes the oscillation.

Yes, the measurements shown were directly at the inputs. I am seeing this problem on two different copies of the same board, although the magnitude and frequency of the oscillations differ by a few hundred mV and a few MHz respectively.

I have tested that the connections are correct already, and they are fine. I don't think there is a problem with the solder joints. I can superimpose an arbitrary signal on top of those oscillations by changing the inputs accordingly.

I've been dealing with the same problem. The stubborn oscillations at 6.8MHz were present in my design. This was even present with input shorted to the ground. I was suspecting the elmag cross-talk between some chip traces and output on the PCB. However, neither moving the pcb traces, nor termination of the inputs by purely resistive load did not have an effect. Finally, I found that the AD8421 went unstable with connected capacitive load . So I put a small 3.3kOhm resistor in series with output and the oscillations have gone away.

I sometimes also observed some super-regenerative oscillations at hundreds of kHz when the inputs were loaded by a capacitance. Again, series resistor with value of about 120Ohm helped. Such a small resistor won't contribute considerably to the noise figure, but will stabilize the overall circuit performance.